What is this article about?

The article by Theeuwes (1992) is about the ability of parallel stage visual processing to selectively guide the attention. Visual information processing consists of two stages. First, the early, preattentive stage, operating without capacity limitations. The attention cannot be selectively drawn to one certain stimulus. The second one, limited-attentive-capacity stage can only deal with one or few items at once. Hereby, the nature of a stimulus makes a difference. For example, when searching for a certain color, the form did not interfere, and when searching for a certain form among stimuli, the color did not interfere.

Different theories describe the processes. One theory describing a bottom-up activation, indicating that parallel stage is followed by focal attention. Other theories focus on top-down processing, indicating that visual attention is always influenced by the knowledge about the target.

In the experiments of this study, the question of whether selectivity can be obtained when not only the stimulus dimension is known (e.g. that the target has a unique color), but also the exact feature value (e.g. the target is green). In order to answer this question three experiment has been conducted.

What has been done in experiment 1a?  

The first experiement had four conditions. The non-distractor color condition, green circles, as a unique color, was embedded in red circles. In the distractor color condition, one red circle was a red square. The non-distractor form condition consisted of green circles and green squares. The distractor form condition was one of the green squares red.

During the tasks, 5,7 or 9 items were located around a fixation point. The participants in the form condition, and those in the color condition received both conditions, with 144 trials each. Each participant should search for the target and press the fitting response key. Speed and accuracy was measured.

Results and Discussion

The results showed if the target item differs in color, the form of the item does not interfere with the reactivity. However, if the form differs, the reactivity is slowed down. Selectivity for color seems to depend on bottom-up activation. Selectivity for top-down activation has not been found. Those results align with previous studies.

What has been done in experiment 1b?

The method and procedure were similar to the conduction of experiment 1a. Eight participants joined 144 trials. The form condition was used, participants had to look for green circle within green squares in a non-distractor and in a distractor condition.

Results and Discussion

Results showed that a top-down control is not possible and practice does not improve selectivity.

What has been done is experiment 2?

The difference in selectivity for form and for colors was tested in experiment 2. In total 16 participants joined the experiment, with eight participants assigned to the form condition and eight to the color condition. The focus of this experiment was on asymmetric selectivity

Results and Discussion

Results have shown that form with is an easier target to select attention on, whereas the search for a different color is more difficult if forms are the same.

How can the findings of all three experiment be summarized?

The goal of the three experiments was to understand the extent to which the first stage, the preattentive stage, can selectively guide attention. Previous studies found that stimulus discrimination towards a dimension does not occur. The current study found that stimulus discrimination towards a feature (form or color) does also not occur. Bottom-up processing seems to be common process. However, this does not exclude a possibility of top-down processing within the first stage.